1. Field of the Invention
The present invention relates to an automobile antenna system, and more particularly, to an automobile antenna system for performing diversity reception.
2. Description of the Prior Art
Antenna systems are indispensable to automobiles which must positively receive various broadcast waves such as those for radio, television and telephone at the receivers located within the vehicle. Such antenna systems are also very important for citizen band transceivers.
However, high-frequency waves in a general VHF band such as FM broadcast waves have a strong tendency to experience rectlinear propagation, so that direct waves and other waves involving reflection from buildings, mountains or other obstacles interfere with each other and cause multipath noise such as distortion or momentary interruption of sound. An automobile antenna system with a single antenna cannot reduce such multipath noise which is produced at the time of reception of waves in a high-frequency band.
Furthermore, it is difficult to receive broadcast waves stably by a conventional pole antenna during the travel of a car, because, due to the directivity of the antenna, the receiving state of the antenna changes in correspondence with the direction in which the broadcast waves come every time the travelling direction of the automobile changes.
To solve this problem, a diversity-reception type antenna system has been proposed, in which two antennas are disposed on the vehicle body at predetermined intervals so that receiving operation is taken over by the antenna which enjoys superior reception by automatic changeover, whereby the directivity of the antenna is improved and the amount of multipath noise is lowered.
However, a conventional antenna is generally a pole antenna which projects outwardly from the vehicle body, and although it is superior in performance in its own way, it always remains a nuisance from the viewpoint of vehicle body design.
Furthermore, the pole antenna is disadvantageous in that it is subject to damage, vandalism or theft and also in that the antenna acts to generate noise during high-speed driving. For such reasons, there has been a strong desire to eliminate the need for such pole antennas.
Especially, when diversity reception is performed by an automobile antenna system, a plurality of antennas are required, as described above, and further, with the enlargement of the frequency bands for broadcast or communication waves received within automobiles, a multiplicity of antennas have been required in accordance with each frequency band.
As a result, the aesthetic appearance of the automobile is disadvantageously damaged, and the receiving performance is greatly deteriorated by electrical interference between antennas.